The present invention relates to current sensors for measuring current flowing in conductors. More particularly, but not exclusively, the invention relates to a current sensor forming part of an alternating current (AC) fiscal electricity meter for measuring the energy supplied by a utility company.
In a published PCT patent application PCT/GB00/02851 (WO 01/11376), there is described a current sensor which responds to a magnetic field generated by a current flowing in a load conductor.
In a published U.S. Pat. No. 4,894,610, there is described a current-transformer arrangement for a static electricity meter.
In a published PCT patent application no. PCT/GB00/02604, there is described an electronic circuit formed on a printed circuit board which is configured as a current-to-voltage converter using a printed circuit track as a sensor, the sensor being placed in parallel with a primary current carrying conductor, for example a bus bar.
Situations arise where a current sensor is required having the following characteristics:
(a) a load current bearing conductor having a simple linear shape;
(b) a sensor scaling factor susceptible to modification merely by altering dimensions of the sensor in one direction;
(c) a circuit board real estate which is used efficiently and is substantially rectangular in format with a minimum number of vias and/or cross-overs; and
(d) virtual immunity to remotely generated alternating magnetic fields.
Current sensors described in the aforesaid PCT applications and US patent are not capable of providing simultaneously all the characteristics (a) to (d) above.
According to the present invention, there is provided a current sensor for measuring current flowing between a power source and a load, the sensor comprising:
an inlet for connection to the source;
an outlet for connection to the load;
conductor paths connected between the inlet and the outlet for guiding currents through the sensor; and
sensing means associated with each conductor path for sensing current flowing therethrough and for generating a corresponding current measure indicative of the magnitude of the current,
characterised in that each sensing means comprises an array of elongate conductive elements which are, firstly, so arranged as to be each mutually dissimilarly coupled to the local magnetic field generated, in use, by current flowing through the associated conductor path and, secondly, so interconnected as to render the sensing means substantially insensitive to magnetic fields generated remotely therefrom.
Such a sensor is capable of one or more of the following:
(a) operating in conjunction with a load current bearing conductor having a simple linear shape;
(b) providing a sensor scaling factor susceptible to modification merely by altering dimensions of the sensor in one direction;
(c) occupying a circuit board real estate which is used efficiently and is substantially rectangular in format with a minimum number of vias and/or cross-overs; and
(d) being substantially immune to remotely generated alternating magnetic fields.
Preferably, the one or more conductive paths are disposed to couple their magnetic fields generated in response to the one or more currents flowing therethrough preferentially to a subset of the zones, thereby generating a residual signal indicative of the magnitude of the one or more currents, the residual signal for use in providing the current measure. Preferentially coupling to a subset of the zones is of advantage in that the zones can be arranged to provide a substantially negligible response to uniform magnetic fields generated by sources remote to the sensor to render the sensor substantially insensitive to such uniform fields, whilst providing measuring sensitivity by virtue of preferentially coupling magnetic fields generated by the one or more currents to the subset of zones.
The one or more conductive paths are preferably implemented as one or more substantially-linear elongate conductive members in proximity of the zones and are arranged to couple magnetically preferentially to one or more of the zones. Incorporation of such substantially-linear conductive members is of advantage in that they are easier to manufacture in comparison to conductive members of complex shape, for example conductive members including circular current paths.
Preferably, the one or more elongate conductive members are integral with the elongate elements on an insulating substrate. Integral mounting of the one or more elongate members onto the substrate is of advantage in that measurement sensitivity of the sensor is dramatically enhanced in comparison to spaced-apart mounting of the elongate members with respect to the substrate.
Alternatively, the one or more elongate conductive members are preferably disposed spatially mutually separated from the elongate elements by a gap region. Such a separated configuration is of advantage in that the sensor is capable of operating with a greater potential difference between the one or more members and the elongate elements.
The zones are preferably implemented as elongate regions comprising coil elements. Such elongate disposition of the elongate regions assists to enhance the measurement sensitivity of the sensor in comparison to a non-elongate implementation of the sensor.
Preferably, to improve manufacturability and reduce cost, the elongate elements are implemented in the form of conductive tracks on a printed circuit board.
On account of the use of a transformation described later, it is feasible to reduce the need for track cross-overs and thereby largely circumvent the need to use circuit board vias. Thus, the elongate elements of the sensor are preferably interconnected on the circuit board substantially without the need to use vias.
More preferably, the printed circuit board is a multi-layer board and the elongate coil elements are duplicated at a plurality of levels in the board and corresponding overlaid zones coupled together in order to enhance current measuring sensitivity of the sensor.
For example, the elongate coil elements are preferably duplicated at several of the layers in the form of a stack and connected in series to provide an increase in output signal magnitude resulting from magnetic fields generated by current flow in the one or more conductive paths coupling into the stack.
Preferably, elongate coil elements of the overlaid zones are interconnected at regions remote from where the one or more load conductors couple magnetically to one or more of the elements. Implementing interconnection at regions remote for active coupling zones of the sensor enables a greater sensor measurement sensitivity to be achieved.
Insensitivity of the sensor to uniform remotely-generated magnetic fields is an important performance characteristic of the sensor. Especially when the sensor is implemented comprising a stack of elongate coil elements, there is tendency for the sensor to become sensitive to uniform magnetic fields incident parallel to the plane of the sensor. By alternately swapping connection of elongate elements in the several layers, it is possible to arrange for EMFs generated by in-plane uniform fields to mutually oppose rendering the sensor substantially immune to such uniform fields. Thus, preferably, the elongate coil elements are connected alternately between layers so as to render the sensor less sensitive to uniform magnetic fields incident on the sensor having field components parallel to the plane of the sensor.
Conveniently, in order to render the sensor more efficient in its use of circuit board real estate, the elongate coil elements are preferably arranged so that their elongate axes are substantially mutually parallel.
In order for the sensor to provide a substantially linear measurement response for a wide range of currents conveyed through the sensor to the load, the elongate coil elements are preferably non-ferromagnetically coupled to the one or more conductive paths. The use of ferromagnetic components in the sensor would impart thereto an upper limit of linear measurement due to ferromagnetic component magnetic saturation.
In cost sensitive applications, it is desirable that the sensor is capable of providing a measure of average current in a plurality of conductive paths. Therefore, the sensor is preferably arranged such that a plurality of the conductive paths are disposed on opposing major faces of the plane encompassing the elongate coil elements, the sensor thereby providing in use an output signal indicative of the summation of the one or more currents flowing in the plurality of conductive paths. By scaling the summation by the number of current paths present, a measure of average current flow in the current paths can be achieved.
The sensor can be supplemented by one or more voltage sensors for power measurement purposes. Therefore, the sensor preferably further comprising voltage sensing means for sensing substantially the voltage of the power source and generating a corresponding voltage measure. A product of the current measure and voltage measure enables a measure of power to be calculated.
When the sensor is used to measure power, the sensor preferably further comprises signal processing means for integrating the current measure with respect to time to provide an integral current measure and computing means to compute a product of the integral current measure and the voltage measure to derive a measure of energy consumed by the load.
In order to render the sensor relatively inexpensive to manufacture when adapted for power sensing applications, the signal processing means and the computing means are preferably integrated onto a single silicon integrated circuit.
Advantageously, the sensor preferably further comprises a memory for storing calibration data for one or more of correcting scale factor, offset and phase errors within the sensor. Such calibration is important for many potential applications for the meter, for example especially when customer billing occurs on the basis of measurements provided by the sensor. Moreover, calibration can often be a time consuming and therefore expensive procedure during sensor manufacture hence recording calibration data in the memory is capable of facilitating more rapid calibration during manufacture. The calibration data can, for example, be subsequently used to scale output from the sensor when the sensor is in operation. More preferably, the memory is also operable to store serial number information and the measure of power consumed by the load. In order that data stored in the memory is not lost when power is removed from the sensor, the memory preferably includes non-volatile memory, for example EEPROM.
Remote interrogation of sensors is an increasingly important contemporary operating requirement. Thus, preferably, the sensor comprises interfacing means for enabling the sensor to be remotely interrogated. More preferably, the interfacing means comprises an Internet connection for enabling the sensor to be interrogated and/or to output measurement data via the Internet.
In a second aspect of the present invention, there is provided an electrical energy meter comprising one or more sensors according to the first aspect of the invention for use in measuring and recording electrical energy consumed.
Preferably, the meter is configured in a 2S-type configuration; the 2S-type configuration is defined later. More preferably, in order to reduce the number of current sensors required, the meter comprises a single sensor according to the first aspect of the invention arranged to provide in use an average current measure of currents flowing through a plurality of load conductors mounted in close proximity to, or in contact with, the sensor. More preferably, the plurality of load conductors are mounted on opposing major faces of the single sensor.
Alternatively, the meter can be in a 12S-type configuration; the 12S-type configuration is defined later.
Alternatively, the meter is preferably implemented as a three-phase meter and is provided with two current sensors and means for deriving a third phase current by way of performing a simultaneous equation solution to signals derived from said two current sensors. By using the solution, for example based on Blondel""s theorem, it is possible to reduce the number of current sensors required and thereby render the meter less expensive to manufacture, simpler in construction and more compact.
In a third aspect of the present invention, there is provided a method of calibrating a sensor according to the first aspect of the present invention, the method comprising the steps of:
(a) applying a supply voltage Vin with substantially zero current drawn from the sensor, and then computing a zero offset power to ensure the power zero accuracy of the sensor;
(b) drawing a known operating current from the sensor to a load and computing a gain calibration constant for ensuring accurate power scale-factor; and
(c) applying a current signal which is substantially reactive, for example with 89xc2x0 degrees between current and voltage vectors, and then computing a phase calibration constant.
Preferably, the zero offset power, the power scale-factor and the phase calibration constant are stored in a non-volatile memory of the sensor.
In a fourth aspect of the present invention, there is provided a method of measuring electrical power using a sensor according to the first aspect of the present invention, the method comprising the steps of:
(a) connecting the sensor to a source of power;
(b) connecting a load to the sensor;
(c) connecting voltage sensing means to the sensor whereat it is connected to the load;
(d) taking a measure of the current delivered through the sensor to the load and a measure of voltage developed across the load; and
(e) calculating a product of the current measure and the voltage measure to provide a measure of power consumed by the load.
Preferably, the method further comprises the step of integrating the power measure to obtain a measure of cumulative power consumed by the load. More preferably, the cumulative measure of power is stored is in a non-volatile memory. Most preferably, the non-volatile memory is an electrically erasable programmable read only memory (EEPROM).
In order to render the method less expensive and simpler to apply, the sensor is preferably configured to measure an average of current flowing in a plurality of paths in close proximity, or in contact with, the sensor.